This application claims priority of German Application No. 102 19 805.5, filed Apr. 30, 2002, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a method for the stabilization of the radiation output of a gas discharge-coupled radiation source in pulsed operation, particularly of an EUV source based on a gas discharge. It is preferably applied in semiconductor chip fabrication for maintaining dose stability in EUV extreme ultraviolet lithography.
b) Description of the Related Art
EUV radiation sources seem to be the most promising development trend for the next generation in lithography machines for semiconductor chip fabrication. Structure widths of less than 50 nm are to be achieved in this connection. It is hoped that the corresponding resolution will be achieved with EUV radiation (around 13.5 nm) and moderate numerical apertures of the applicable optics of 0.2 . . . 0.3.
The EUW radiation source itself represents the component group with the most limitations and resulting sources of error. It is comprising the EUV radiation output, the predetermined solid angle in which the radiation is detectable, the magnitude of the emitting plasma, the pulse repetition frequency, pulse-to-pulse fluctuations (output fluctuations and spatial fluctuations of EUV emission), the life of the discharge unit, and the life of the optical components.
A complete lithography system is judged principally on the basis of throughput (i.e., wafers processed per hour), image quality and cost of ownership (CoO).
The in-band radiation output arriving at the wafer location and the sensitivity of the photo-sensitive layer (resist) in the desired EUV range determine the throughput of the system, while the image quality is determined by the dose stability (dose accuracy). According to V. Banine et al. (Proc. SPIE Vol. 3997 (2000) 126), this dose accuracy is essentially determined by the following three contributing factors:
a) pulse quantization
b) pulse-to-pulse stability
c) spatial stability of the emitting plasma.
Pulse quantization is scanner-specific. The quantity of beam pulses that can fall into the slit of the scanner can vary in that the first and/or last pulse does not strike the slit. This results in a dose error of Da:
Da≅2/N2,
where N is the number of pulses falling into the slit and is composed as follows:
N=slit width * repetition rate/scanning speed.
Contributors b) and c) are source-specific. Contributor b) is given by the pulse energy fluctuations of the emitting plasma.
Db≅3"sgr"(energy)/N1/2.
For example, 3"sgr" (energy) is the maximum amplitude of the energy fluctuations.
Further, a spatial fluctuation of the plasma results from the statistical character of the gas discharge. This likewise causes an error contribution to the radiation dose effective at the target location (e.g., wafer) in a manner predetermined by the optics of the scanner:
Dc≅6*3"sgr"(spatial)/N1/2.
The substantial disadvantage of known EUV radiation sources consists in that the contributions of energy fluctuations in EUV emissions resulting from different causes is not detected and evaluated through measurement and the EUV sources are therefore unregulated at least insofar as the constancy of the pulse energy is concerned.
It is the primary object of the invention to find a new possibility for stabilizing the radiation output of a gas discharge-coupled radiation source in pulsed operation, particularly an EUV radiation source, which allows the pulse energy to be regulated on the basis of pulse energy fluctuations of the radiation emission detected through measurements without requiring regular calibration measurements of the pulse energy depending on the charging voltage (a recalibration of the so-called E(U) curve).
According to the invention, in a method for the stabilization of the radiation output of a gas discharge-coupled radiation source in pulsed operation, particularly an EUV source, in which a constant pulse energy is to be radiated within a sequence of many consecutive radiation pulses, the above-stated object is met by the following steps:
measurement of the pulse energy of every radiation pulse of the EUV source,
measurement of at least one influencing variable for every radiation pulse,
formation of time averages of the pulse energy and influencing variable,
calculation of the deviation of the actual measurement values from the average values of the pulse energy and influencing variable,
determination of the ratio of pulse energy and influencing variable at least for an operating point in which the pulse energy is to be kept constant, wherein the ratio is assumed to be linear based on the small magnitude of the fluctuations compared to the averages and is determined in the form of a proportionality factor of the functional dependence,
determination of scaled correlation coefficients of the measured influencing variables in order to determine the degree of statistical dependence of the influencing variables upon the pulse energy,
regulation of the pulse energy by means of PI regulation based on the actual proportionality factor (a) corresponding to at least one influencing variable with the greatest correlation coefficient.
The charging voltage at the high-voltage source for the gas discharge is advantageously measured as primary influencing variable. Other influencing variables which can be measured and evaluated in a supplementary manner or in combination with one another are the breakdown voltage of the gas discharge measured, e.g., at discharge capacitors which are arranged for the buildup of a reproducible discharge voltage between high-voltage source and discharge electrodes (so-called peaking capacitors), the position of the emitting plasma (measured by means of a spatially resolving detector), or the pressure of the working gas measured in the discharge chamber of the radiation source.
The pulse energy, as regulating variable, is advisably measured by means of an energy detector in the immediate vicinity of the emitting plasma. This main energy detector is preferably arranged opposite to an illumination beam path directed to a target object. The pulse energy can be measured at the target object location (e.g., a wafer in an exposing device for semiconductor lithography) as another (supplemental) influencing quantity.
Due to the degradation of mirror surfaces (e.g., of the collector optics for bundling the emitted EUV beam) and the like aging processes of the main energy detector caused by radiation, a reference pulse energy is advantageously measured at greater intervals in a low-light reference beam path in addition to the pulse energy of every individual beam pulse in order to compensate for degradation phenomena.
The proportionality factors in a regulation that takes into account the dependency of pulse energy on several of the influencing variables mentioned above are advantageously calculated by statistical evaluation of the measured pulse energy and of the selected influencing variables (X, Y, . . . ), wherein the selected influencing variables are included according to the general equation for different (two, in this case) interference variables (X, Y) of a regulating process
E=a X+b Y, 
where
a=( less than EX greater than xe2x88x92b less than XY greater than )/ less than X2 greater than 
and
b=( less than EY greater than xe2x88x92a less than XY greater than )/ less than y2 greater than 
and the equations can be expanded in a corresponding manner when required.
The proportionality factors a and b are preferably calculated by applying the least squares method for a direct dependence between the pulse energy and the influencing variables.
In order to determine how pulse energy and influencing variables (the latter are seen in this case as interference variables in a regulating process) are correlated with one another, the correlation coefficients are advisably calculated (for two different interference quantities X and Y) as
K1(E,X)= less than XE greater than /( less than X2 greater than  less than E2 greater than )1/2
K2(E,Y)= less than YE greater than /( less than Y2 greater than  less than E2 greater than )1/2
K3(X,Y)= less than XY greater than /( less than X2 greater than  less than Y2 greater than )1/2.
These correlation coefficients can then also be advantageously used for in-situ analysis of changes in the EUV radiation source itself and in the illumination beam path to the target object (e.g., wafer) and for in-situ analysis of errors and interference in the radiation source in its entirety
For regulation of fast changes in the pulse energy, it is advisable to carry out the control of the gas discharge-coupled radiation source by changing the charging voltage at the high-voltage source (power supply). In this connection, it is advantageous for a fast regulation of pulse energy to use a PI (proportional-integral) regulation in the stationary operating regime, i.e., within a sequence of beam pulses in which constant discharge conditions can be assumed (e.g., burst of several hundred pulses) and to calculate a proportionality factor required for this from a statistical analysis of deviations of the regulating quantity and influencing variable(s) from their averages.
In slowly changing processes, the regulation of another input quantity, e.g., pressure p in the discharge chamber, would also be possible.
For the regulation of comparatively slow changes ( greater than 1 s) of the pulse energy, it can also be advantageous to vary the pressure of the working gas in the discharge chamber. A pressure regulation is possible by way of the Xe throughput or He throughput with time constant 0.5 . . . 1 s and, therefore, a pressure regulation for suitable determination of the operating point is reasonable.
The basic idea of the invention is based on the understanding that the fluctuations of pulse energy of gas discharge-coupled radiation sources in pulsed operation which can not be tolerated particularly in EUV radiation sources for semiconductor lithography are brought about by a variety of influencing variables (interference variables) without their relationship to the regulating variable pulse energy or to one another being suitably taken into account.
In contrast to the regulation of radiation energy by way of the rise in the function of the pulse energy depending upon the charging voltage, which is known from excimer lasers, the invention proceeds from a statistical analysis of the deviations of the influencing variables (interference variables) from their averages by way of a large number of radiation pulses with continuous measurement and regulation. In this connection, linear dependencies can be specified due to the small quantity of the deviations compared to the associated averages. Accordingly, a simple evaluation of the correlation of the influencing variables can be used for proportional regulation of the pulse energy.
With the method according to the invention, it is possible to detect pulse energy fluctuations in the radiation emitted by a gas discharge-coupled radiation source in pulsed operation based on different influencing variables that are detected by measuring techniques and to select the essential influencing variables and suitable control variables for regulation through statistical correlation so as to achieve a sufficient pulse-to-pulse stability of the radiation, particularly for EUV radiation sources, and a high dose stability at the target point (wafer).
The invention will be described more fully in the following with reference to embodiment examples.